In electrophotographic image formation, an electrostatic latent image is formed on an electrostatic latent image bearing member such as a photoconductive material, and formed into a toner image with a charged toner. The toner image is then transferred onto and fixed on a recording medium to thereby form output image. In the field of electrophotography, full-color copiers and printers have been rapidly brought to the mainstream in place of monochrome copiers and printers recently. Therefore, a market of the full-color copiers and printers has tended to expand.
In full-color image formations, generally, three color toners of yellow, magenta and cyan, or four color toners of black as well as yellow, magenta, and cyan are superimposed to thereby reproduce all colors. Thus, in order to obtain a sharp full-color image being excellent in color-reproducibility, it is necessary to smooth the surface of a fixed toner image so as to reduce light scattering. For this reason, in conventional full-color copiers, images having a middle-glossiness to a high-glossiness of 10% to 50% have been often formed.
Generally, as a method of fixing a dry-toner image on a recording medium, the contact-heat fixing method has been often employed in which a roller or belt having a smooth surface is allowed to press-contact with a toner while heating the roller or belt. This method has advantages in that it exhibits high-thermal efficiency, enables high-speed fixing and imparts glossiness and transparency to color toner images. On the other hand, this method inconveniently causes a so-called offset phenomenon in which a part of a toner image adheres to the surface of a fixing roller and then transferred onto another image, because the surface of a heat-fixing member is made in contact with a molten toner under pressure and then they are separated from each other.
In order to prevent the offset phenomenon, there has been adopted a method of forming a surface layer of a fixing roller with use of a material being excellent in releasing property such as a silicone rubber or a fluoro-resin, and further applying a toner adhesion preventing oil such as a silicone oil onto the surface layer of the fixing roller. This method is extremely effective in preventing toner-offset. However, this method requires additionally providing a device for supplying the oil, leading to an increase in size of the fixing device.
Therefore, in monochrome image formations, there has been often adopted an oil-less system in which a toner, which has a high viscoelasticity in a molten state and contains a releasing agent in order to avoid internal fracture of the molten toner, is used to eliminate the need for applying oil onto the fixing roller, or a system in which the toner is used to extremely decrease the application amount of oil.
Meanwhile, also in full-color image formations, an oil-less system has tended to be employed for decreasing in size of a fixing device and simplifying the structure thereof similarly to in monochrome image formations. However, in full-color image formations, there is a need to reduce the viscoelasticity of the toner in a molten state in order to smooth the surface of a fixed toner image. Therefore, the full-color image formations more easily cause the offset phenomenon than the non-glossy monochrome image formations, which makes it difficult to employ the oil-less system in the full-color image formations. When a releasing agent is incorporated into a toner, the toner is increased in adhesivity, so that transferability of the toner to a recording medium is degraded. Further, the incorporation of the releasing agent into the toner disadvantageously causes toner filming, leading to degradation in chargeability and thus in durability.
On the other hand, there have been various attempts to prolong service life of a carrier by coating the core surface of the carrier with a resin having a low-surface energy such as a fluoro-resin or a silicone resin, for the purpose of preventing the toner filming from occurring, forming a uniform carrier surface, preventing a carrier surface from being oxidized, preventing moisture-sensitivity from reducing, prolonging service life of a developer, preventing a carrier from adhering onto the surface of a photoconductor, protecting a photoconductor from being scratched or abraded, controlling charge polarity, and adjusting the charge amount.
Examples of the carrier coated with the resin having a low-surface energy include a carrier coated with a room temperature curable silicone resin and a positively charged nitrogen resin (see PTL 1), a carrier coated with a coating material containing at least one modified silicone resin (see PTL 2), a carrier having a coating layer containing a room temperature curable silicone resin and a styrene-acrylic resin (see PTL 3), a carrier in which the surface of a core particle is coated with two or more layers of a silicone resin so that the layers do not adhere to each other (see PTL 4), a carrier in which the surface of a core particle is coated with multiple layers of a silicone resin (see PTL 5), a carrier of which surface is coated with a silicone resin containing a silicon carbide (see PTL 6), a positively charged carrier coated with a material exhibiting critical surface tension of 20 dyn/cm or less (see PTL 7), and a carrier coated with a coating material containing fluoroalkyl acrylate (see PTL 8).
Recently, however, there has been increasingly a demand for higher speed, reduction in environmental waste load resulting from prolonging service life, and reduction in cost for printing per page in image forming apparatus. Therefore, there is a need for a carrier having higher durability.
On the other hand, resistivity is an important property for a carrier. The resistivity of the carrier is controlled so as to achieve an intended print quality depending on a system of image forming apparatus which is used in combination with the carrier. A coating layer of the carrier contains electroconductive particles as a material for controlling the resistivity. Exemplary examples of the electroconductive particles include carbon black, titanium oxide, zinc oxide, and ITO (indium tin oxide). Among them, a single-particle type carbon black and ITO coated with an electroconductive layer have been used as excellent electroconductive particles in many cases. For example, a carrier in which carbon black is used as electroconductive particles has been described (see PTL 9, PTL 10, and PTL 11). However, there is a need for improvement in the above carrier because it has not responded to a recent image forming under high stressed conditions, so that problematic color smear has been occurred.
Also, electroconductive particles in which base particles are coated with ITO serving as an electroconductive material have been described (see PTL 12, PTL 13, PTL 14, PTL 15, and PTL 16). However, in the case of the electroconductive particles in which base particles are coated with thin layers of the electroconductive material being excellent in electroconductive performance, when a carrier is formed therefrom and used in high-speed image forming apparatus, the thin layers of the electroconductive material which is exposed on the surfaces of carrier particles are scraped off due to collision of the carrier particles with each other within a developing device. As a result, the base particles having high hardness are rapidly exposed, so that resin coating layers in the carriers is acceleratedly decreased in impact resistance, further leading to scraping of the coating layers and decreasing in resistivity. Accordingly, carrier scattering occurs, which makes it impossible for the carrier to be used over a long period of time.
As such, in order to achieve a high-durable carrier, the option of electroconductive particles and a coating resin is selected is important.